1. Field of the Invention
The present invention relates to a differential limiting device of the type that the transmission torque of a pilot clutch is amplified by a cam mechanism to bring a main clutch into friction engagement.
2. Discussion of the Related Art
Heretofore, as differential limiting devices for vehicles, there has been known one which is constructed by combining a planetary gear mechanism and an electromagnetic pilot clutch mechanism, as described in Japanese unexamined, published patent application No. 9-144845 (144845/1997).
The differential limiting device described in the patent document is provided with the planetary gear mechanism which is composed of planetary gears in meshing with a sun gear and a ring gear and a housing rotatably receiving the planetary gears in receiving bores thereof. The differential limiting device is further provided with a main clutch arranged between the ring gear and the sun gear. In the differential limiting device, the relative rotation between the housing and the ring gear causes connection means (i.e., the pilot clutch mechanism) to operate, whereby the main clutch is brought into operation to limit or suppress the differential rotation between the sun gear and the ring gear.
Thus, the transmission torque from an engine is transmitted to the housing, and the transmission torque transmitted to the housing is distributed by the planetary gear mechanism to the sun gear and the ring gear. Further, the main clutch is brought into friction engagement by the operation of the electromagnetic pilot clutch mechanism to limit the differential rotation between the ring gear and the sun gear.
The differential limiting device of the aforementioned type is mounted on a drive power transmission system of a vehicle and is used to constitute the vehicle as a four-wheel drive vehicle. In the use of the differential limiting device under this circumstance, it is sometimes the case that reversal occurs in the differential rotation while torque transmission is being performed with the cam mechanism being operated by the pilot clutch. The occurrence of the reversal in the differential rotation causes the differential limiting device or devices around the same to generate a strange noise.
For example, in a four-wheel drive vehicle which employs the aforementioned differential limiting device to distribute the transmission torque from an engine to front and rear wheels, the reversal takes place between the rotational speed of the housing to which the rotation from the engine is transmitted and the rotational speed of the sun gear to which the rotation of the front wheels is transmitted, or the rotational speed of the ring gear to which the rotation of the rear wheels is transmitted. That is, the reversal takes place when the vehicle transitions from the acceleration state to the deceleration state or from the deceleration state to the acceleration state. When the reversal occurs in the differential rotation between the wheels and the engine, the cam rotational angle of the cam mechanism, frictions among the component members and the like may occasionally cooperate to generate the strange noise.
The cause may be analyzed as follows. That is, when a reversal occurs in the cam mechanism, the pressuring force exerted on the main clutch is lost, and the main clutch thus comes not to generate the friction torque. When the differential rotation further proceeds, the main clutch again receives the pressuring force from the cam mechanism and revives to generate a predetermined torque. Where the time continues long during which the main clutch does not generate any friction torque, the rotational speed difference becomes large between the idle rotations of main outer clutch plates and main inner clutch plates, and a large difference in rotational energy is produced therebetween. When the main clutch is restored to the state in which it again transmits the torque, the large difference in energy is absorbed within a very short period of time, which causes the strange noise to be generated at the cam mechanism.
Another differential limiting device described in U.S. Pat. No. 6,378,677 (Japanese unexamined, published patent application No. 2002-106605) has also been known as one which is designed to prevent the generation of such a strange noise. In this patent, it is described at column 11, line 44 through column 12, line 52 that the generations of a strange noise and a shock can be prevented as follows. That is, the backlash at spline engagement portions of a main clutch is made to be larger than the backlash at a spline engagement portion of a cam mechanism. Thus, when the reversal in the differential rotation causes the reversal of the cam mechanism in motion transmission, the reversal motion of the cam mechanism is completed earlier than the main clutch makes an idle rotation through the backlash at the spline engagement portions thereof.
However, where the backlash at the spline engagement portions of the main clutch is made to be larger as described in the aforementioned United States patent, clutch plates of the main clutch are moved in the circumferential direction when not in friction engagement, and this results in generating the strange noise and vibration. Moreover, the amounts have to be controlled of those backlashes which are provided at the spline engagement portions of all the components which compose the main clutch and the cam mechanism. However, it is difficult and impractical in production control to control the backlash amounts of all the portions.